Rapid development of memory storage devices achieved in recent years demands an increase in recording density and capacity of such devices. This, in turn, requires an improvement in the device's track per inch (TPI) characteristic. In order to improve the TPI of a memory storage device, for example a hard disk drive, better performance characteristics, such as a spindle motor speed, rotational accuracy, noise level, shock resistance, etc., should be achieved. With respect to the bearing systems utilized in hard disk drives, an improvement in a rotational vibration and ability to withstand high-speed rotation has been sought.
To respond to industry's demand for the improvement in rotational vibration, ball bearing systems have been manufactured having rotating elements made of materials having low adhesion and superior wear resistance. Specifically, ball bearing systems have been manufactured with rotating elements made of silicone nitride ceramics. Additionally, the machining accuracy of all bearing elements, i.e., the inner and outer rings and rotary members, is continued being improved upon.
The other known solution to the above demand for even greater speeds, is the use of fluid dynamic pressure bearings as a replacement for conventional ball bearings. Fluid dynamic bearings are lower in noise and allow to dramatically improve rpm speed of spindle motors used in hard disk drives.
In these fluid dynamic pressure bearings, a lubricating fluid (for example, oil or air) is filled into a gap formed between a shaft and a support member, typically a sleeve. Dynamic pressure generated in the lubricating fluid by a relative rotation of the shaft and the support member causes the shaft to be suspended away from the inner walls of the support member, thus forming a hydrodynamic bearing.
To facilitate generation of the hydrodynamic pressure and formation of the bearing, pressure-generating grooves may be provided the shaft and/or the support member. To form pressure-generating grooves, a pattern of sideways V-shaped or herringbone-shaped grooves is etched into the bearing surface. In the formation of these sideways V-shaped pressure-generating grooves, an electrolytic machining method is used to form the requisite shape, dimensions and surface condition on the relevant workpiece surface by imparting an electrochemical dissolving action to the workpiece surface to be machined. Therefore, machining accuracy, improvement of productivity, efficiency, etc. of the electrolytic machining method have become the new issues in the bearing manufacturing process.
Here, before explaining the details of the electrolytic machining method, we shall first briefly explain the structure of a spindle motor having a rotatable shaft supported for rotation by a fluid dynamic pressure bearing.
As shown in FIG. 5, spindle motor 01 is provided with rotor hub 03 that rotates relative to base 02. Fluid dynamic pressure bearing 030 is interposed between base 02 and rotor hub 03. Rotary shaft 020 is vertically mounted to rotor hub 03 and is inserted into bearing sleeve 031 secured to the inner perimeter surface of interior cylindrical wall 014 of base 02. Lubricant is supplied to a sliding portion of bearing sleeve 031 and rotary shaft 020. When the shaft rotates, the fluid dynamic pressure is generated in the lubricant due to the action of herringbone-shaped dynamic pressure-generating groove 033 formed on the inner surface of the sleeve 031. Formed dynamic pressure causes rotary shaft 020 to float upward from bearing sleeve 031. Thrust plate 034 is mounted at the bottom of rotary shaft 020. Although not shown in detail, dynamic pressure-generating herringbone shaped grooves may be formed on the upper end surface of a thrust ring 034 and/or the step surface of bearing sleeve 031 which opposes this upper end surface. Further, herringbone-shaped dynamic pressure-generating grooves may be formed on the lower end surface of the thrust plate and/or on the upper surface of counter plate 037 fit into the bottom end of bearing sleeve 031. Lubricant is supplied between the step of bearing sleeve 031 and the upper surface of thrust plate 034, and between the lower surface of the thrust plate and counter plate 037. When rotary shaft 020 rotates, the lubricant pressure rises due to the action of the (herringbone-shaped) dynamic pressure-generating groove, the thrust ring 034 mounted on the rotary shaft 020 floats up from the counter plate 037, rotating in a non-contacting state in a position midway between the sleeve 031 step surface and the counter plate 037 inner surface, not contacting the step surface of the sleeve 031.
The electrolytic machining method used in the invention of this application is a machining method for obtaining a specified shape, dimension and surface condition of a groove by concentrating or limiting the electrolytic dissolving action on a specified part of a workpiece in the course of machine forming the above-described dynamic pressure-generating groove.
Prior technology for this electrolytic machining method is depicted in FIG. 4. FIG. 4 is a diagram showing the overall construction of an electrolytic machining device for the purpose of forming a dynamic pressure-generating groove on the inner perimeter surface of a hollow cylindrical workpiece (sleeve) OW used as a support member for a spindle motor shaft.
A positive terminal of the machining pulsed power supply 012 is connected to the workpiece OW, while a negative terminal is connected to tool 016, such that a DC pulsed voltage is applied between machining surface 015 of the workpiece OW and tool 016.
Tool 016 is provided with electrode 017 having sideways V-shaped (herringbone shaped) projecting portions approximately corresponding in shape and dimensions to the grooves which will be formed on the target surface. The size of sideways V-shaped (herringbone shaped) projecting portions of electrode 017 is usually somewhat smaller than the size of a desired dynamic pressure-generating groove. Tool 016 with projecting electrode 017 is inserted into the inner bore of the workpiece OW such that during electrolytic machining the electrode faces the target machining surface 015 across a tiny gap. Target machining surface 015 will later form the inner circumferential surface of the bearing sleeve. The workpiece OW is accurately positioned for the electro-machining process using stationary fixture 013 and is secured to machining bed 02.
Electrolyte 011 stored in an electrolytic tank 07 is directed to the tiny gap provided between target machining surface 015 of the workpiece OW and the outer surface of tool 016. Electrolyte 011 is configured so as to be supplied in a fixed amount to the provided tiny gap by pump 08 through filter 09. Electrolyte 011 that has passed through this tiny gap is then returned to electrolytic tank 07. Returned electrolyte 011 is again supplied through circulation by means of pump 08.
During electrolytic machining, pump 08 operates and electrolyte 011 is continuously supplied to the aforementioned gap from electrolyte tank 07. Here, when electrolyte 011 passes through the gap between target machining surface 015 of the workpiece OW and tool 016, products of electrolysis mix into electrolyte 011. In electrolytic machining, current densities are extremely high and machining gaps are extremely small, so the occurrence of electrolytic products and heating of electrolyte 011 have a significant effect on the quality of machining. Thus, there is a danger that if these effects are not quickly removed, machining may not progress. It is therefore necessary for the flow speed of electrolyte 011 to be fast, and, while there may be variation depending on machining conditions, it is generally desirable for the flow speed to be in the range of 2–10 m/sec. When the electrolysis products are sedimentary, recirculating electrolyte 011 should be used only after cleaning. To separate and remove electrolytic products from the electrolyte 011 in the electrolyte tank 07 centrifuging, precipitation, filtering, or a combination of all of these may be used.
In a device thus constituted, current flows from the pulsed power supply 012 and between the workpiece OW and tool 016 when a DC voltage (pulsed voltage) is applied between the workpiece OW and tool 016 for a specified time (electrolytic machining time). Assuming, for example, that the electrolyte is a sodium nitrate (NaNO3) fluid and the workpiece OW surface is nickel (Ni), then primarily the following electrolytic reaction occurs:    Workpiece surface: Ni→Ni+++2e−    Tool surface: 2Na+=2H2O=2NaOH=H2 
When the workpiece OW is Fe, the following type of reaction occurs:
Workpiece surface: Fe→Fe+++2e−
                Fe++=2OH−→Fe(OH)2             Tool surface: 2H++2e−→H2 
In this type of reaction, the surface material of the workpiece OW facing tool 016 dissolves in the electrolyte 011, and a dynamic pressure-generating groove having a shape corresponding to the projecting pattern of the projecting electrode 017 is formed on target machining surface 015 of the workpiece OW.
During electrolytic machining, the current flowing between the workpiece OW and the tool 016 is controlled to be turned off and on as necessary for electrolytic machining. The shape and dimensions of the desired dynamic pressure-generating groove are determined by setting machining conditions such as the size of the gap between target machining surface 015 of the workpiece OW and tool 016. Other electrolytic machining conditions that may influence the shape and dimensions of the grooves include the material of the workpiece OW, the applied current (A) and the pulsed voltage application time (T). A dynamic pressure-generating groove of the required shape is thus formed in target machining surface 015 of the workpiece OW.
In currently utilized electro-machining processes, a different machining bed, electrolyte tank, and pulsed power supply have to be provided for each workpiece type. Therefore, it is currently not possible to line up workpieces of the same type or differing types and machine them simultaneously. Therefore, machining has to be performed one item at a time, resulting in poor productivity. It is also currently necessary to adjust settings for each workpiece type that is very time consuming. Furthermore, each time the shape of the workpiece or the shape of the desired dynamic pressure-generating groove is changed, additional time is required for changeover to set the new precise gap width. Therefore, from a productivity and cost standpoint, market demands are not adequately met by currently available electro-machining methods.
An individually set up electrolyte tank is erected on each machining device, and, because electrolyte tanks differ from one machining device to another even for the same workpiece, machining similar workpieces on different devices may take place under different electrolyte concentrations and states of electrolyte deterioration. This means that electrolytic concentration has to be controlled separately for each electrolyte tank.